1. Field of the Invention
The present invention is directed to a local gradient coil arrangement for a nuclear magnetic resonance tomography apparatus having an essentially hollow-cylindrical geometry and composed of a number of gradient coils for different spatial directions, the gradient coil arrangement being separable at least at one side along a separating parting line proceeding in the axial direction of the arrangement.
2. Description of the Prior Art
As known, the spatial resolution of the nuclear magnetic resonance signals in nuclear magnetic resonance tomography ensues by superimposing a magnetic field gradient on a uniform, static, basic field on the order of magnitude of 1 T. The principles of such imaging are set forth, for example, in the article by Bottomley, "NMR Imaging Techniques and Applications: A Review", in Review of Scientific Instrumentation 53, 9, 9/82, pages 1319-1337.
U.S. Pat. No. 4,910,462 discloses a gradient coil for generating a longitudinal magnetic field. This coil is etched from a planar structure and is then rolled into a cylindrical shape. In order to avoid having to connect a number of conductors to one another after rolling the etched structure into a cylindrical shape, the conductors are arcuately returned at the ends of the flat structure. When being rolled to form a cylinder, these returns overlap such that respective conductors having opposite current directions lie on top of one another so that a disturbing, resultant magnetic field is not produced.
German OS 39 32 648 discloses a high-frequency local coil for nuclear magnetic resonance tomography that is divisible for easier introduction of body parts.
European Application 0 313 213 discloses a gradient coil arrangement wherein the gradient field strength in the examination subject is increased by means of attaching at least one set of gradient coils so as to be closer to the body of the examination subject than the other coils. A departure is thus made from the otherwise rotationally symmetrical form.
The gradient coils are usually arranged on a carrying tube which is rigidly installed in a basic field magnet and which encloses the entire patient opening. Given a radius r of the gradient coil at a given field strength and field quality, the energy of the gradient coil, and thus its inductivity, is proportional to r.sup.n, whereby n&gt;5. For this reason, it is especially advantageous to keep the diameter of gradient coils as small as possible, i.e., to place them as close as possible to the subject to be examined. Given rigidly installed gradient coils, however, physical limits as to the minimum diameter are imposed by the required patient space.
For this reason, so-called local gradient coils were proposed that do not surround the entire patient space but, in the form of special coils, surround only the actual anatomy to be examined, i.e., for example, the head or extremities. Similar to local radio-frequency coils, interchangeable special gradient coils are thus provided for different examination regions. The advantage of lower inductivity associated with the smaller volume of such local gradient coils is especially significant when fast rise and decay times of the gradient current or high gradient field strengths are desired. This is the case given fast pulse sequences such as, for example, echo planar imaging (EPI).
When all of the required gradient coils for the three spatial directions are applied on a hollow cylinder, this must be passed over the patient, or the patient must be inserted into the hollow cylinder. This is rather impractical in terms of manipulation, especially since the hollow cylinder should be implemented as snugly as possible.
If, however, the gradient coil is made divisible (separable) for easier manipulation, then the problem arises that the individual turns of the gradient coil must be connected to one another via a plurality of contact points. This is explained in greater detail below with reference to FIGS. 1 through 3. FIGS. 1 through 3 respectively show the same hollow cylinder 5 on which three gradient coils 2 through 4 for the three spatial directions are applied. For greater clarity, the three gradient coils 2 through 4 are respectively separately shown in FIGS. 1 through 3.
FIG. 1 shows a saddle-shaped gradient coil of a conventional type having individual coils 2a through 2d for generating a gradient in the y-direction according to the Cartesian coordinate system entered in FIG. 1. The inner conductors thereby supply the useful field, the conductors proceeding parallel to the z-directions make no contribution to the y-gradient due to their direction, and the conductors lying at the outside can be left out of consideration due to their greater distance from the examination region in the center of the hollow cylinder 5. A separating line 5a is shown in FIG. 1 for the hollow cylinder 5. The hollow cylinder 5 with the gradient coils 2 can be unproblematically separated along this line 5a since no conductor crosses the separating line.
FIG. 2 shows a gradient coil 3 for the z-direction. This is composed of two coil parts 3a and 3b with opposite current directions. Given a separation of the hollow cylinder 5, all turns would have to be interrupted here.
FIG. 3 shows a gradient coil 4 for the x-direction. Like the gradient coil for the y-direction, this is composed of four individual saddle coils; only the saddle coils 4a and 4b are visible in FIG. 3. The gradient coil for the x-direction is offset by 90.degree. in azimuthal direction compared to the gradient coil 2 for the y-direction. All turns thus also cross the separating line 5a given these coils and thus must be divided.
A number of contacts are therefore required at the separating line 5a when one wishes to generate the magnetic field gradients for all spatial directions with local gradient coil. These contacts must have a considerable current-carrying capability since gradient currents on the order of magnetic of, typically, 200 through 250 A are required. Further, high voltages also occur at the gradient coils. Finally, the contacts would have to be easy to open. Such solutions would be extremely involved in technical terms insofar as they are even possible.